Effect of Stacking Order in Cylindrical Geometry on the Growth of ZnAl2O4 Spinel Phase

Growth kinetics of ZnAl2O4 spinel phase was followed in double layered nanotubes (DLNTs), i.e. in cylindrical geometry in two different layer sequences. DLNTs were annealed at 700 °C in ambient atmosphere for different times. ZnAl2O4 phase formed due to solid-state reaction. To follow the growth kinetics of the spinel, thin lamellae were prepared from the cross sections of the tubes and studied in a transmission mode scanning electron microscope (TSEM). The phase grows parabolically with time, independently of the layer sequence. The growth rate was higher in the )ZnO)Al2O3) layer sequence, than for the reverse case. This can be explained by the competing vacancy fluxes induced by mechanical stress developing during the solid-state reaction and by the difference in the diffusion fluxes of the two constituents. This is the first case that the influence of closed geometry and layer sequence on spinel growth between two oxide layers has been experimentally investigated.